Printed circuit boards (PCBs), multi-chip modules (MCMs), and similar structures comprising multiple integrated circuits mounted upon their surfaces are used extensively in modern electronic devices and systems. PCBs typically contain multiple conductive and dielectric layers interposed upon each other, and interlayer conductive paths, referred to as vias, which may extend from an integrated circuit mounted on a surface of the PCB to one or more conductive layers embedded within the PCB. MCMs and other similar structures typically have similar configuration and structure (e.g. a substrate comprising dielectric and conductive layers having inter-layer vias). For ease of reference, all such structures shall hereafter be referred to as "boards".
The speed and complexity of integrated circuits are increased rapidly as integrated circuit technology advances from very large scale integrated (VLSI) circuits to ultra large scale integrated (ULSI) circuits. As the number of components per chip, the number of chips per board, the modulation speed and the degree of integration continue to increase, electrical interconnects are facing fundamental limitations in areas such as speed, packaging, fan-out, and power dissipation. Multi-chip module (MCM) technology has been employed to provide higher clock speeds and circuit density. However, conventional technologies based on electrical interconnects fail to provide requisite multi-Gbits/sec clock speed in intra-MCM and inter-MCM applications.
Additionally, a printed circuit board may, in some instances, be quite large and the conductive paths therein can be several centimeters in length. As conductive path lengths increase, impedances associated with those paths also increase. This has a detrimental effect on the ability of the system to transmit high speed signals. It is therefore desirable that impedances of conductive paths be minimized; in order to, for example, transmit high speed signals in the 1 Gb/sec range.
For these reasons, a conductive layer having relatively high impedance can be replaced by an optical waveguide, which can transmit signals at the speed of light. Waveguides are particularly beneficial when transmitting high speed signals over relatively long distances, as signal loss is minimized.
While embedded waveguides may be formed in a board or semiconductor substrate, difficulties arise when converting electrical signals emanating from an integrated circuit, mounted on the board's surface, to optical signals within the embedded waveguide. Some conventional conversion schemes employ light emitting lasers as transmitters and photo-detectors as receivers, mounted on the upper surface of a board adjacent bonding pads/sockets, which receive integrated circuit pins. The electrical signal from an output pin of an integrated circuit is transmitted, via a conductor at or above the board's surface, to the light emanating laser; which then converts the electrical signal to optical energy. That optical energy permeates from the board surface, through several layers of the board, downward to a waveguide embedded within the board. A grating coupler is typically placed within the waveguide to receive the optical energy and directionally transmit an appropriate wave through the waveguide; eventually to be received by an optical receiver distally located from the grating. An optical receiver can be placed proximate to another integrated circuit, separate from the integrated circuit initiating the transmitted optical signal. The optical receiver can then receive the optical energy, converting it to an electrical signal to be transmitted to an input pin of the receiving integrated circuit.
Thus, using an optical waveguide enhances the speed at which signals can be transmitted between integrated circuits. However, inefficiencies in transmitting optical energy through several layers of conductive and non-conductive materials within a board limit the light-to-electrical and electrical-to-light (opto-electronic) coupling efficiency; thereby limiting high-speed signal transmission within a system.
Additionally, conventional opto-electronic interconnect systems are typically incompatible with commercial manufacturing processes utilizing boards. Consider, for example, a printed circuit board used as a motherboard within a personal computer. A motherboard manufacturer will typically, if not exclusively, use automated equipment and processes to mount desired semiconductor devices on the surface of a printed circuit board. Opto-electronic devices often require care in handling and processing that standard semiconductor devices do not. Therefore, use of conventional opto-electronic interconnect systems will either require modification of standard manufacturing processes or additional processing steps to account for the presence or addition of opto-electronic components on the board surface. Additional monetary and time costs resulting from use of conventional opto-electronic interconnect systems thus render these approaches commercially unviable.
Therefore, a system providing opto-electronic signal communications fully embedded and planarized within a printed circuit board or substrate is now needed; providing opto-electronic interconnection and bus structures compatible with existing surface mount and board production technologies, and providing enhanced opto-electronic coupling efficiency enabling high system operational speeds, while overcoming the aforementioned limitations of conventional methods.